Field of the Invention
The present invention relates to a multilayer ceramic capacitor whose ceramic grains that constitute dielectric layers have a roughly uniform concentration distribution of specified donor elements.
In recent years, there is a high demand for smaller electronic components to support higher density electronic circuits used in mobile phones, tablet terminals, and other digital electronic devices, and accordingly multilayer ceramic capacitors (MLCC) that constitute these circuits are rapidly becoming smaller in size and larger in capacitance.
The capacitance of a multilayer ceramic capacitor is directly proportional to the dielectric constant of the constituent material of the dielectric layers constituting the capacitor and also to the number of the dielectric layers, and inversely proportional to the thickness of one dielectric layer. For this reason, answering the demand for smaller multilayer ceramic capacitors requires that the dielectric constant of their material be increased, while the thickness of the dielectric layer be reduced to increase the number of layers.
However, reducing the thickness of the dielectric layer increases the voltage applied per unit thickness, which in turn shortens the service life of the dielectric layer and consequently reduces the reliability of the multilayer ceramic capacitor. Accordingly, dielectric compositions to which such donor elements as Mo and W are added to improve the service life, are proposed.
In addition, the distribution of abundance ratios of the aforementioned donor elements and other additive elements in the ceramic grains constituting the dielectric layer affects the performance of the multilayer ceramic capacitor. About this point, Patent Literature 1, for example, describes a dielectric ceramic offering improved dielectric breakdown voltage, wherein the dielectric ceramic is such that additive elements such as Mn, V, Cr, Co, Ni, Fe, Nb, Mo, Ta, and W are distributed roughly uniformly over the entire range from the grain boundary to the center of the crystal grain. In the example of this literature, barium carbonate, titanium oxide, and oxides of the aforementioned additive elements are mixed together and calcinated at 1200° C., after which other additive element compounds are added and the resulting mixture is calcinated further at 1000° C. in an oxidizing ambience, and thereafter the obtained ceramic material mixture is used to prepare green sheets that are then laminated together and sintered at 1200° C. in a reducing ambience for 2 hours, and then heat-treated at 600° C. in an oxidizing ambience for 30 minutes, to obtain a laminated capacitor. The distribution of additive elements in the dielectric ceramic of the laminated capacitor thus obtained is specified as roughly uniform, as mentioned above, but in reality there was a difference of around seven times between the grain boundary portion and the center portion, according to the literature.
Patent Literature 2 proposes a multilayer ceramic capacitor whose service life will not be shortened due to dielectric breakdown, etc., even if the number of dielectric layers is increased or the thickness of the dielectric layer is reduced, and which also permits size reduction and capacitance increase; wherein the constitution of the multilayer ceramic capacitor is such that its ceramic grain is constituted by a crystalline core and a shell that encloses the core, that additive elements such as Mn, V, Cr, Mo, Fe, Ni, Cu, and Co are added to the core, and that the concentration of these additive elements increases from the center of the core toward the shell. In the example of this literature, barium carbonate, titanium oxide, and compounds of the aforementioned additive elements are mixed together and then calcinated at 200° C. for 2 hours to synthesize barium titanate containing the additive elements, after which other additive elements are added and the resulting mixture is calcinated at 1000° C. for 2 hours to obtain ceramic grains, and thereafter using the obtained ceramic grains ceramic green sheets are prepared and laminated together and then sintered at 1130° C. in a reducing ambience for 3 hours and heated at 600° C. in an oxidizing ambience for 30 minutes, to obtain a multilayer ceramic capacitor. It is indicated that, with the obtained multilayer ceramic capacitor, the concentration of additive elements in the core of the ceramic grain that forms the dielectric layer was around 290 ppm, while the concentration of additive elements in the shell was around 410 ppm.
Also, Patent Literature 3 describes a barium titanate ceramic grain as a dielectric ceramic that will give a multilayer ceramic capacitor offering good capacitance-temperature characteristics and excellent service life characteristics, wherein such grain is characterized in that it has a core and a shell and contains rare earth elements R and M (here, M represents at least one type selected from the group consisting of Mg, Mn, Ni, Co, Fe, Cr, Cu, Al, Mo, W, and V) as secondary constituents, where the total concentration of R and M slopes from the grain boundary to the core and becomes minimal in a portion and maximal in another portion.